(q) how to carry out and interpret the results of the following chemical…

The idea

Each test detects a specific molecule through a colour change. Biuret: add an equal volume of biuret solution (sodium hydroxide plus dilute copper(II) sulfate) to the sample; a purple colour indicates peptide bonds, so protein is present, while a blue colour is negative. Benedict's: add Benedict's reagent (blue copper(II) sulfate) and heat in a water bath; a brick-red precipitate shows a reducing sugar. For a non-reducing sugar, first boil with dilute hydrochloric acid to hydrolyse it, neutralise with sodium hydrogencarbonate, then repeat Benedict's; a positive result after hydrolysis but not before indicates a non-reducing sugar such as sucrose. Iodine: add iodine in potassium iodide solution; blue-black indicates starch, orange-brown is negative. Emulsion: add ethanol and shake, then pour into water; a cloudy white emulsion indicates lipid.

What earns the marks

• Biuret test: add biuret solution to the sample; purple indicates protein (peptide bonds present), blue is a negative result.
• Benedict's test for reducing sugar: add Benedict's reagent and heat in a water bath; a brick-red precipitate indicates a reducing sugar.
• Non-reducing sugar: boil with dilute acid to hydrolyse, neutralise with sodium hydrogencarbonate, then repeat Benedict's; positive only after hydrolysis indicates a non-reducing sugar.
• Iodine test: add iodine in potassium iodide solution; blue-black indicates starch, orange-brown is negative.
• Emulsion test: add ethanol and shake, then pour into water; a cloudy white emulsion indicates lipid.
• Interpretation requires comparing the observed colour or precipitate with the known positive and negative outcomes for each test.

Common mistakes and how to put them right

• Assuming Benedict's reagent detects all sugars directly.

  Put it right: It detects reducing sugars only; non-reducing sugars need acid hydrolysis and neutralisation first.

• Forgetting to neutralise the acid before adding Benedict's reagent after hydrolysis.

  Put it right: Sodium hydrogencarbonate must be added, or the acid prevents the colour change.

• Confusing the iodine positive colour with the emulsion result.

  Put it right: Iodine gives blue-black with starch; the emulsion test gives a cloudy white emulsion with lipid.

• Treating a blue biuret result as positive.

  Put it right: Blue is negative; purple indicates protein.

Exam tips

• Learn the reagent, the condition (for example heating for Benedict's) and the positive colour for each test as a matched set.
• For non-reducing sugars, state the full sequence: hydrolyse with acid, neutralise, then repeat Benedict's.
• When interpreting, always state what the observed colour shows about the molecule present, not just the colour name.